Method and apparatus for insitu vapor generation

ABSTRACT

A method of forming an oxide on a substrate. According to the method of the present invention a substrate is placed in a chamber. An oxygen containing gas and a hydrogen containing gas are then fed into the chamber. The oxygen containing gas and the hydrogen containing gas are then caused to react with one another to form water vapor in the chamber. The water vapor then oxidizes the substrate.

The present application is a Continuation of prior application Ser. No.09/507,946 now U.S. Pat. No. 6,159,866 filed Feb. 22, 2000 which is aContinuation of prior application Ser. No. 09/033,391 filed Mar. 2, 1998now U.S. Pat. No. 6,037,273 which is a Continuation-in-Part of priorU.S. patent application Ser. No. 08/893,774, filed Jul. 11, 1997 nowabandoned and assigned to the present Assignee.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of steam oxidation and morespecifically to a method and apparatus for insitu moisture generation ina rapid thermal steam oxidation process.

2. Discussion of Related Art

In the fabrication of modem integrated circuits, such as microprocessorsand memories, oxidation processes are used to passivate or oxidizesilicon films. A popular method to oxidize silicon surfaces and filmssuch as polysilicon gate electrodes and substrates is to use a steam(H₂O) oxidation process. In such cases water vapor (H₂O) is brought intoan oxidation chamber to react with the silicon surfaces to form silicondioxide.

Present steam oxidation processes generally take place in multi-waferresistively heated “hot wall” furnaces. Present steam oxidationprocesses typically use a pyrogenic torch or bubbler located outside ofthe reaction chamber in which the steam oxidation process is to takeplace. In the case of a pyrogenic torch, a hydrogen containing gas andan oxygen containing gas are ignited by a flame in a reaction area atatmospheric pressure and located away from and generally in a differentchamber than the chamber in which wafers are placed. The flame ignitionoccurs at atmospheric pressure. A problem associated with pyrogenictorch methods, is that for safety reasons only certain concentrationratios of hydrogen containing gas and oxygen containing gas can beutilized. Limiting. the available gas ratio unduly restricts onesability to generate ambients with desired concentrations of H₂O/H₂ orH₂O/O₂. For example, in order to keep a stable flame burning, torchmethods typically require H₂:O₂ ratios of more that 0.5:1 and less than1.8:1, respectively. Bubblers are also undesirable for moisturegeneration in that they can be a significant source of contamination andbecause they cannot accurately and reliably control the amount ofmoisture generated.

Another problem associated with the use of pyrogenic torches andbubblers is that these methods are not easily implemented into modernrapid thermal heating apparatuses which utilize light sources for rapidtemperature ramps and reaction times measured in terms of seconds asopposed to minutes and hours. Rapid thermal heaters are preferred overresistively heated furnaces because of their excellent temperatureuniformity and control provides more for uniform processing and becausetheir short reaction times reduce the thermal budget of fabricateddevices.

Thus, what is desired is a method and apparatus for generating moisturein a rapid thermal heating apparatus which does not suffer fromcontamination and safety issues and which can use a full spectrum of gasmixtures as well as concentration ratios.

SUMMARY OF THE INVENTION

A method of forming an oxide on a substrate is described. According tothe present invention a substrate is placed in a reaction chamber. Anoxygen containing gas and a hydrogen containing gas are then fed intothe chamber. The oxygen containing gas and the hydrogen containing gasare then caused to react in the chamber to form water vapor. The formedwater vapor is used to oxidize the substrate. In an embodiment of thepresent invention a combined partial pressure of the oxygen containinggas and the hydrogen containing gas in the reaction chamber is developedbetween 1-50 Torr to enhance the oxidation rate of silicon by the watervapor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a rapid thermal heating apparatus which canimplement the insitu moisture generation oxidation process of thepresent invention.

FIG. 2 is an illustration of the light source placement in the rapidthermal heating apparatus of FIG. 1.

FIG. 3 is a flow chart which illustrates a rapid thermal oxidationprocess which utilizes the insitu moisture generation process of thepresent invention;

FIG. 4a is a cross sectional view of a semiconductor wafer or substrateprior to steam oxidation.

FIG. 4b is an illustration of a cross sectional view showing theformation of an oxide on the substrate of FIG. 4a by a rapid thermaloxidation process which utilizes insitu moisture generation of thepresent invention.

FIG. 5 is a graph which illustrates the detonation pressure created forvarious O₂/H₂ concentration ratios having a partial pressure of 150torr.

FIG. 6 illustrates plots which depict oxide thickness versus reactantgas partial pressure for different H₂/O₂ concentrations.

FIG. 7 is a plot which illustrates oxide thickness versus H₂/O₂ reactantgas concentration ratios.

FIG. 8 illustrate plots which depict oxide thickness versus oxidationtime for various concentration ratios and reactant gas partialpressures.

FIG. 9 is a plot which illustrates oxide thickness versus total flow ofprocess gas.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention describes a novel method and apparatus for insitumoisture generation in a rapid thermal steam oxidation process. In thefollowing description numerous specific details such as apparatusconfigurations as well as process specifics such as time and temperatureare set forth in order to provide a thorough understanding of thepresent invention. One skilled in the art will appreciate the ability touse alternative configurations and process details to the disclosedspecifics without departing from the scope of the present invention. Inother instances well known semiconductor processing equipment andtechniques have not been described in detail in order to notunnecessarily obscure the present invention.

The present invention describes a steam oxidation process. According tothe present invention, steam (H₂O) is formed in the same chamber aswhich the substrate to be oxidized is located (i.e., steam is formedinsitu with the substrate). According to the present invention areactant gas mixture comprising a hydrogen containing gas, such as butnot limited to H₂ and NH₃ and an oxygen containing gas, such as but notlimited to O₂ and N₂O, is fed into a reaction chamber in which asubstrate is located. The oxygen containing gas and the hydrogencontaining gas are caused to react to form moisture or steam (H₂O) inthe reaction chamber. The reaction of the hydrogen containing gas andthe oxygen containing gas is ignited or catalyzed by heating the waferto a temperature sufficient to cause the moisture reaction Because theheated wafer is used as the ignition source for the reaction, themoisture generation reaction occurs in dose proximity to the wafersurface. Reactant gas concentrations and partial pressures arecontrolled so as to prevent spontaneous combustion within the chamber.By keeping the chamber partial pressure of the reactant gas mixture atless than or equal to 150 torr during the reaction, any reactant gasconcentration may be utilized to form moisture without causingspontaneous combustion. The insitu moisture generation process of thepresent invention preferably occurs in a reduced pressure single waferchamber of a rapid thermal processor. A rapid thermal steam oxidationprocess utilizing insitu moisture generation is ideally suited foroxidizing a silicon film or substrate in the formation of modern ultrahigh density integrated circuits.

The insitu moisture generation process of the present invention ispreferably carried out in a rapid thermal heating apparatus, such as butnot limited to, the Applied Materials, Inc. RTP Centura with a HoneycombSource. Another suitable rapid thermal heating apparatus and its methodof operation is set forth in U.S. Pat. No. 5,155,336 assigned to theAssignee of the present application. Additionally, although the insitumoisture generation reaction of the present invention is preferablycarried out in a rapid thermal heating apparatus, other types of thermalreactors may be utilized such as the Epi or Poly Centura single wafer“cold wall” reactor by Applied Materials used to form high temperaturefilms (HTF) such as epitaxial silicon, polysilicon, oxides and nitrides.

FIGS. 1 and 2 illustrate a rapid thermal heating apparatus 100 which canbe used to carry out the insitu moisture oxidation process of thepresent invention. Rapid thermal heating apparatus 100, as shown in FIG.1, includes an evacuated process chamber 13 enclosed by a sidewall 14and a bottom wall 15. Sidewall 14 and bottom wall 15 are preferably madeof stainless steel. The upper portion of sidewall 14 of chamber 13 issealed to window assembly 17 by “O” rings 16. A radiant energy lightpipe assembly 18 is positioned over and coupled to window assembly 17.The radiant energy assembly 18 includes a plurality of tungsten halogenlamps 19, for example Sylvania EYT lamps, each mounted into a light pipe21 which can be a stainless steel, brass, aluminum or other metal.

A substrate or wafer 61 is supported on its edge in side chamber 13 by asupport ring 62 made up of silicon carbide. Support ring 62 is mountedon a rotatable quartz cylinder 63. By rotating quartz cylinder 63support ring 62 and wafer 61 can be caused to rotate. An additionalsilicon carbide adapter ring can be used to allow wafers of differentdiameters to be processed (e.g., 150 mm as well as 200 mm). The outsideedge of support ring 62 preferably extends less than two inches from theoutside diameter of wafer 61. The volume of chamber 13 is approximatelytwo liters.

The bottom wall 15 of apparatus 100 includes a gold coated top surface11 for reflecting energy onto the backside of wafer 61. Additionally,rapid thermal heating apparatus 100 includes a plurality of fiber opticprobes 70 positioned through the bottom wall 15 of apparatus 100 inorder to detect the temperature of wafer 61 at a plurality of locationsacross its bottom surface. Reflections between the backside of thesilicon wafer 61 and reflecting surface 11 create a blackbody cavitywhich makes temperature measurement independent of wafer backsideemissivity and thereby provides accurate temperature measurementcapability.

Rapid thermal heating apparatus 100 includes a gas inlet 69 formedthrough sidewall 14 for injecting process gas into chamber 13 to allowvarious processing steps to be carried out in chamber 13. Coupled to gasinlet 69 is a source, such as a tank, of oxygen containing gas such asO₂ and a source, such as a tank, of hydrogen containing gas such as H₂.Positioned on the opposite side of gas inlet 69, in sidewall 14, is agas outlet 68. Gas outlet 68 is coupled to a vacuum source, such as apump, to exhaust process gas from chamber 13 and to reduce the pressurein chamber 13. The vacuum source maintains a desired pressure whileprocess gas is continually fed into the chamber during processing.

Lamps 19 include a filament wound as a coil with its axis parallel tothat of the lamp envelope. Most of the light is emitted perpendicular tothe axis towards the wall of the surrounding light pipe. The light pipelength is selected to at least be as long as the associated lamp. It maybe longer provided that the power reaching the wafer is notsubstantially attenuated by increased reflection. Light assembly 18preferably includes 187 lamps positioned in a hexagonal array or in a“honeycomb shape” as illustrated in FIG. 2. Lamps 19 are positioned toadequately cover the entire surface area of wafer 61 and support ring62. Lamps 19 are grouped in zones which can be independently controlledto provide for extremely uniform heating of wafer 61. Heat pipes 21 canbe cooled by flowing a coolant, such as water, between the various heatpipes. The radiant energy source 18 comprising the plurality of lightpipes 21 and associated lamps 19 allows the use of thin quartz windowsto provide an optical port for heating a substrate within the evacuativeprocess chamber.

Window assembly 17 includes a plurality of short light pipes 41 whichare brazed to upper/lower flange plates which have their outer edgessealed to an outer wall 44. A coolant, such as water, can be injectedinto the space between light pipes 41 to serve to cool light pipes 41and flanges. Light pipes 41 register with light pipes 21 of theilluminator. The water cooled flange with the light pipe pattern whichregisters with the lamp housing is sandwiched between two quartz plates47 and 48. These plates are sealed to the flange with “O” rings 49 and51 near the periphery of the flange. The upper and lower flange platesinclude grooves which provide communication between the light pipes. Avacuum can be produced in the plurality of light pipes 41 by pumpingthrough a tube 53 connected to one of the light pipes 41 which in turnis connected to the rest of the pipes by a very small recess or groovein the face of the flange. Thus, when the sandwiched structure is placedon a vacuum chamber 13 the metal flange, which is typically stainlesssteel and which has excellent mechanical strength, provides adequatestructural support. The lower quartz window 48, the one actually sealingthe vacuum chamber 13, experiences little or no pressure differentialbecause of the vacuum on each side and thus can be made very thin. Theadapter plate concept of window assembly 17 allows quartz windows to beeasily changed for cleaning or analysis. In addition, the vacuum betweenthe quartz windows 47 and 48 of the window assembly provides an extralevel of protection against toxic gasses escaping from the reactionchamber.

Rapid thermal heating apparatus 100 is a single wafer reaction chambercapable of ramping the temperature of a wafer 61 or substrate at a rateof 25-100° C./sec. Rapid thermal heating apparatus 100 is said to be a“cold wall” reaction chamber because the temperature of the wafer duringthe oxidation process is at least 400° C. greater than the temperatureof chamber sidewalls 14. Heating/cooling fluid can be circulated throughsidewalls 14 and/or bottom wall 15 to maintain walls at a desiredtemperature. For a steam oxidation process utilizing the insitu moisturegeneration of the present invention, chamber walls 14 and 15 aremaintained at a temperature greater than room temperature (23° C.) inorder to prevent condensation. Rapid thermal heating apparatus 100 ispreferably configured as part of a “cluster tool” which includes a loadlock and a transfer chamber with a robotic arm.

A method of insitu generation of moisture or steam in a rapid thermaloxidation process according to the present invention is illustrated inflow chart 300 of FIG. 3. The method of the present invention will bedescribed with respect to an insitu moisture generation process in therapid thermal heating apparatus illustrated in FIGS. 1 and 2.Additionally, the oxidation process of the present invention will bedescribed with respect to the steam oxidation or passivation of asilicon gate electrode 402 and a silicon substrate surface 404 of asilicon wafer 61 as shown in FIG. 4a. It is to be appreciated that theinsitu moisture generation oxidation process of the present inventioncan be used to oxidize any form of silicon including epitaxial,amorphous, or polycrystalline, including doped and undoped forms.Additionally the process can be used to passivate or oxidize otherdevice or circuit features including but not limited to emitter andcapacitor electrodes, interconnects and trenches, as well as be used toform gate dielectric layers.

The first step according to the present invention, as set forth in block302, is to move a wafer or substrate, such as wafer 61 into vacuumchamber 13. As is typical with modern cluster tools, wafer 61 will betransferred by a robot arm from a load lock through a transfer chamberand placed face up onto silicon carbide support ring 62 located inchamber 13 as shown in FIG. 1. Wafer 61 will generally be transferredinto vacuum chamber 13 having a nitrogen (N₂) ambient at a transferpressure of approximately 20 torr. Chamber 13 is then sealed.

Next, as set forth in block 304, the pressure in chamber 13 is furtherreduced by evacuating the nitrogen (N₂) ambient through gas outlet 70.Chamber 13 is evacuated to a pressure to sufficiently remove thenitrogen ambient. Chamber 13 is pumped down to a prereaction pressureless than the pressure at which the insitu moisture generation is tooccur, and is preferably pumped down to a pressure of less than 1 torr.

Simultaneous with the prereaction pump down, power is applied to lamps19 which in turn irradiate wafer 61 and silicon carbide support ring 62and thereby heat wafer 61 and support ring 62 to a stabilizationtemperature. The stabilization temperature of wafer 61 is less than thetemperature (reaction temperature) required to initiate the reaction ofthe hydrogen containing gas and oxygen containing gas to be utilized forthe insitu moisture generation. The stabilization temperature in thepreferred embodiment of the present invention is approximately 500° C.

Once the stabilization temperature and the prereaction pressure arereached, chamber 13 is backfilled with the desired mixture of processgas. The process gas includes a reactant gas re comprising two reactantgasses: a hydrogen containing gas and an oxygen containing gas, whichcan be reacted together to form water vapor (H₂O) at temperaturesbetween 400-1250° C. The hydrogen containing gas, is preferably hydrogengas (H₂), but may be other hydrogen containing gasses such as, but notlimited to, ammonia (NH₃), deuterium (heavy hydrogen) and hydrocarbonssuch as methane (CH₄). The oxygen containing gas is preferably oxygengas (O₂) but may be other types of oxygen containing gases such as butnot limited to nitrous oxide (N₂O). Other gasses, such as but notlimited to nitrogen (N₂), may be included in the process gas mix ifdesired. The oxygen containing gas and the hydrogen containing gas arepreferably mixed together in chamber 13 to form the reactant gasmixture.

In the present invention the partial pressure of the reactant gasmixture (i.e., the combined partial pressure of the hydrogen containinggas and the oxygen containing gas) is controlled to ensure safe reactionconditions. According to the present invention, chamber 13 is backfilledwith process gas such that the partial pressure of the reactant gasmixture is less than the partial pressure at which spontaneouscombustion of the entire volume of the desired concentration ratio ofreactant gas will not produce a detonation pressure wave of apredetermined amount. The predetermined amount is the amount of pressurethat chamber 13 can reliably handle without failing. FIG. 5 is a graphwhich shows detonation pressures for different reactant gas mixtures ofO₂ and H₂ at a partial pressure of 150 torr for the spontaneouscombustion of the entire volume, about 2 liters, of chamber 13 at aprocess temperature of 950° C. According to the present invention,insitu moisture generation is preferably carried out in a reactionchamber that can reliably handle a detonation pressure wave of fouratmospheres or more without affecting its integrity In such a case,reactant gas concentrations and operating partial pressure preferably donot provide a detonation wave greater than two atmospheres for thespontaneous combustion of the entire volume of the chamber.

By controlling the chamber partial pressure of the reactant gas mixturein the present invention any concentration ratio of hydrogen containinggas and oxygen containing gas can be used including hydrogen richmixtures utilizing H2/O2 ratios greater than 2:1, respectively, andoxygen rich mixtures using H₂/O₂ ratios less than 0.5:1, respectively.For example, FIG. 5 shows that any concentration ratio of O₂ and H₂ canbe safely used as long as the chamber partial pressure of the reactantgasses is maintained at less than 150 torrs at process temperature. Theability to use any concentration ratio of oxygen containing gas andhydrogen containing gas enables one to produce an ambient with anydesired concentration ratio of H₂/H₂O or any concentration ratio ofO₂/H₂O desired. Whether the ambient is oxygen rich or dilute steam orhydrogen rich or dilute steam can greatly affect device electricalcharacteristics. The present invention enables a wide variety ofdifferent steam ambients to be produced and therefore a wide variety ofdifferent oxidation processes to be implemented.

In some oxidation processes, an ambient having a low steam concentrationwith the balance O₂ may be desired. Such an ambient can be formed byutilizing a reactant gas mixture comprising 10% H₂ and 90% O₂. In otherprocesses, an ambient of hydrogen rich steam (70-80% H₂/30-20%H₂O) maybe desired. A hydrogen rich, low steam concentration ambient can beproduced according to the present invention by utilizing a reactive gasmix comprising between 5-20% O₂ with the remainder H₂ (95-80%). It is tobe appreciated that in the present invention any ratio of hydrogencontaining gas and oxygen containing gas may be utilized because theheated wafer provides a continual ignition source to drive the reaction.Unlike pyrogenic torch methods, the present invention is not restrictedto specific-gas ratios necessary to keep a stable flame burning.

Next, as set forth in block 308, power to lamps 19 is increased so as toramp up the temperature of wafer 61 to process temperature. Wafer 61 ispreferably ramped from the stabilization temperature to processtemperature at a rate of between 10-100° C./sec with 50° C./sec beingpreferred. The preferred process temperature of the present invention isbetween 600-1150° C. with 950° C. being preferred. The processtemperature must be at least the reaction temperature (i.e., must be atleast the temperature at which the reaction between the oxygencontaining gas and the hydrogen containing gas can be initiated by wafer61) which is typically at least 600° C. It is to be noted that theactual reaction temperature depends upon the partial pressure of thereactant gas mixture as well as on the concentration ratio of thereactant gas mixture, and can be between 400° C. to 1250° C.

As the temperature of wafer 61 is ramped up to process temperature, itpasses through the reaction temperature and causes the reaction of thehydrogen containing gas and the oxygen containing gas to form moistureor steam (H₂O). Since rapid thermal heating apparatus 100 is a “coldwall” reactor, the only sufficiently hot surfaces in chamber 13 toinitiate the reaction is the wafer 61 and support ring 62. As such, inthe present invention the moisture generating reaction occurs near,about 1 cm from, the surface of wafer 61. In the present invention themoisture generating reaction is confined to within about two inches ofthe wafer, or about the amount at which support ring 62 extends past theoutside edge of wafer 61. Since it is the temperature of the wafer (andsupport ring) which initiates or turns “on” the moisture generationreaction, the reaction is said to be thermally controlled by thetemperature of wafer 61 (and support ring 62). Additionally, the vaporgeneration reaction of the present invention is said to be “surfacecatalyzed” because the heated surface of the wafer is necessary for thereaction to occur, however, it is not consumed in the reaction whichforms the water vapor.

Next, as set forth in block 310, once the desired process temperaturehas been reached, the temperature of wafer 61 is held constant for asufficient period of time to enable the water vapor generated from thereaction of the hydrogen containing gas and the oxygen containing gas tooxidize silicon surfaces or films to form SiO₂. Wafer 61 will typicallybe held at process temperature for between 30-120 seconds. Process timeand temperature are generally dictated by the thickness of the oxidefilm desired, the purpose of the oxidation, and the type andconcentrations of the process gasses. FIG. 4b illustrates an oxide 406formed on wafer 61 by oxidation of silicon surfaces 402 and 404 by watervapor (H₂O) generated by the insitu moisture generation process of thepresent invention. It is to be appreciated that the process temperaturemust be sufficient to enable the reaction of the generated water vaporor steam with silicon surfaces to form silicon dioxide.

Next, as set forth in block 312, power to lamps 19 is reduced or turnedoff to reduce the temperature of wafer 61. The temperature of wafer 61decreases (ramps down) as fast as it is able to cool down (at about 50°C./sec.). Simultaneously, N2 purge gas is fed into the chamber 13. Themoisture generation reaction ceases when wafer 61 and support ring 62drop below the reaction temperature. Again it is the wafer temperature(and support ring) which dictates when the moisture reaction is turned“on” or “off”.

Next, as set forth in block 314, chamber 13 is pumped down, preferablybelow 1 torr, to ensure that no residual oxygen containing gas andhydrogen containing gas are present in chamber 13. The chamber is thenbackfilled with N₂ gas to the desired transfer pressure of approximately20 torr and wafer 61 transferred out of chamber 13 to complete theprocess. At this time a new wafer may be transferred into chamber 13 andthe process set forth in flow chart 300 repeated.

At times it may be desirable to utilize concentration ratios of hydrogencontaining gas and oxygen containing gas which will produce an ambientwith a large concentration of water vapor (e.g., >40% H₂O). Such anambient can be formed with a reactant gas mixture, for example,comprising 40-80% H₂/60-20% O₂. A gas mixture near the stoichiometricratio may yield too much combustible material to enable safe reactionconditions. In such a situation, a low concentration gas mixture (e.g.,less than 15% O₂ in H₂ ) can be provided into the reaction chamberduring step 306, the wafer temperature raised to the reactiontemperature in step 308, and the reaction initiated with the lowerconcentration ratio. Once the reaction has been initiated and theexisting reactant gas volume begins to deplete, the concentration ratiocan be increased to the desired level. In this way, the amount of fuelavailable at the start of the reaction is kept small and safe operatingconditions assured.

In an embodiment of the present invention a relatively low, reactive gaspartial pressure is used for insitu steam generation in order to obtainenhanced oxidation rates. It has been found that providing a partialpressure of between 1 Torr to 50 Torr of hydrogen gas (H₂) and oxygengas (O₂) that an enhanced oxide growth rate of silicon can be achieved.That is, for a given set of process conditions (i.e., H₂/O₂concentration ratio, temperature, and flow rate) the oxidation rate ofsilicon is actually higher for lower partial pressures (1-50 Torr) of H₂and O₂ than for higher partial pressures (i.e., from 50 Torr to 100Torr).

The plots of FIG. 6 illustrate how reactant gas partial pressures canenhance the oxidation rate of silicon. Plot 602 depicts different oxidethicknesses that are formed for different reactant gas partial pressuresfor an ambient created by reacting 9% H₂ with 91% O₂ at 1050° C. for 30seconds. Plot 604 depicts different oxide thicknesses that are formedfor different reactant gas partial pressures for an ambient created byreacting 33% H₂ with 66% O₂ at 1050° C. for 60 seconds.

As is apparent from the graphs of FIG. 6, as the reactant gas partialpressure of H₂ and O₂ is incrementally decreased from atmosphericpressure to about 50 Torr for 9% H₂, and to about 30 Torr for 33% H₂,the oxidation rate of silicon also decreases incrementally. A decreasein oxidation rate for silicon with a decrease in reactant gas partialpressure is expected in that one would expect when less O₂ and H₂ areavailable for the generation of steam the oxidation rate would decrease.When a reactant gas partial pressure of less than or equal toapproximately 50 Torr for 9% H₂ and 30 Torr for 33%H₂ obtained, however,the oxidation rate quite unexpectedly begins to increase withincremental decreases in reactant gas partial pressure. The oxidationrate continues to increase until a maximum enhanced oxidation rate isreached at approximately 8-12 Torr at which point the oxidation ratebegins to decrease for incremental decreases in reactant gas partialpressure. Although the oxidation rate begins to decrease after themaximum enhanced oxidation rate achieved at 8-12 Torr, it still providesan enhanced oxidation rate (i.e., provides an oxidation rate greaterthan the oxidation rate generated at approximately 50 Torr (9% H₂) and30 Torr (33% H₂)) until a reactant gas partial pressure of approximately1-3 Torr at which point the oxidation rate enhancement falls off.

In the enhanced oxidation embodiment of the present invention, theinsitu steam generation is carried out at a reactant gas partialpressure of oxygen containing gas and hydrogen containing gas whereenhanced oxidation occurs. That is, in an embodiment of the presentinvention insitu steam generation occurs at a maximum reactant gaspartial pressure of oxygen gas (O₂) and hydrogen gas (H₂) which is lessthan or equal to the reactant gas partial pressure at which a decreasein reactant gas partial pressure for a given set of process parameterscauses an increase in the oxidation rate of silicon. Additionally in theenhanced oxidation embodiment of the present invention the minimumreactant gas partial pressure is that at which the oxidation rate isgreater than or equal to the oxidation rate at the maximum reactant gaspartial pressure. The minimum reactant gas partial pressure is generallyabout 1-3 Torr. Enhanced oxidation of silicon by a steam ambient can becreated by reacting H₂ and O₂ between a minimum reactant gas partialpressure of between 1-3 Torr and a maximum reactant gas partial pressureof about 50 Torr. In another embodiment of the present invention insitusteam generation is carried out at a combined oxygen containing gas andhydrogen containing gas partial pressure of between 5-5 Torr which iswhere the peak enhanced oxidation rate occurs.

It is to be appreciated that operating the insitu steam generationprocess of the present invention at a reactant gas partial pressure atwhich enhanced oxidation-occurs is valuable for a number of reasons. Anincrease in oxidation rate means less oxidation time is required to growan oxide of a given thickness which increases throughput which therebydecreases the cost of ownership of a tool. Such increases in waferthroughput are extremely important when single wafer reactors such asrapid thermal heating apparatus 100 are utilized. Additionally shortoxidation times reduce the thermal budget of semiconductor chips, whichimproves their performance and reliability. Additionally, an increase inoxidation rate enables the insitu steam generation process of thepresent invention to be used for the generation of thick oxides (e.g.,oxides greater than 100 Å).

Still further operating at low reactant gas partial pressures not onlyprovides the advantage of enhanced oxidation rate but also provides theadvantage of safe operating conditions in that the detonation pressurecreated by the spontaneous combustion of the entire volume of thechamber is minimized due to the small amount of fuel available.Additionally, operating at low partial pressures prevents thecondensation of moisture inside a “cold wall” chamber which prevents theintroduction of an uncontrolled reactant.

Although the oxidation rate of only two concentration ratios of H₂/O₂are illustrated in FIG. 6, the oxidation rate of other concentrationratios between 2% H₂/98% O₂ to 66% H₂/33% O₂ behave similarly. It hasbeen found that when operating at reactant gas partial pressures whereenhanced oxidation occurs, that the oxidation rate of silicon isinfluenced by the concentration ratio of the hydrogen containing gas andthe oxygen containing gas. For example, FIG. 7 illustrates differentoxidation thicknesses for different concentration ratios of H₂ and O₂for a given set of process parameters (ie., O₂ flow 10 SLM, reactant gaspartial pressure 10 Torr, temperature. 1050° C., and time 30 seconds).As illustrated in FIG. 7, the greatest increase in oxidation rate occursbetween 1-5% H₂ while after 33% H₂ the oxidation rate stabilizes atabout 150 Å per minute.

FIG. 8 illustrates how oxide thickness varies for oxidation time fordifferent insitu steam oxidation processes (33% H₂/66% O₂; 5% H₂/95% O₂;2% H₂/98% O₂, or at 10 Torr) and different dry oxidation processes (100%O₂ at 10 Torr and 100% O₂ at atmospheric). As illustrated in FIG. 8,reduced pressure steam oxidation processes provide for increasedoxidation rates over dry oxidation processes at the same pressure.Additionally, insitu steam generated oxidation processes with a H₂concentration greater than 3% provide higher oxidation rates than do dryoxidation processes at all oxidation pressures including atmosphericpressure.

In an embodiment of the present invention, a concentration ratio Between2% H₂/98% O₂ to 33% H₂/66% O₂ is utilized because such produces asufficient oxidation rate but yet utilizes a low concentration ofreactant gas which makes the process safe. It is to be appreciated thatwhen concentration ratios are doser to the stoichiometric ratio (66%H_(2/)33%O₂) there is the potential for the entire volume of the chamberto spontaneously combust. By operating in the concentration range ofbetween 2%-33% H₂, one is able to obtain oxidation rates near theoxidation rate of the stoichiometric ratio but without the danger of thespontaneous combustion of the entire volume since only a smallpercentage of H₂ is available for reaction. It has been found that byoperating with a concentration ratio of 33% H₂/66% O₂, a good oxidationrate can be obtained while providing a sufficiently low concentration ofH₂ to ensure safe operating conditions.

When operating at oxidation pressures which obtain enhanced siliconoxidation rates, the oxidation rate is strongly influenced by the totalflow rate of the oxygen containing gas and the hydrogen containing gas.For example, FIG. 9 illustrates how the oxidation rate of silicon variesfor the total flow rate of a 33% H₂/66% O₂ reactant gas mix at areactant gas partial pressure of 10 Torr and a temperature of 1050° C.in rapid thermal processing apparatus 100, having a chamber volume ofapproximately 2 liters. As shown in FIG. 9, when operating at lowreactant gas partial pressures, in order to generate enhanced oxidationrates, an increase in the total flow increases the oxidation rate. Asshown in FIG. 9 the oxidation rate increases dramatically for anincrease in total flow when the total flow is less than 10 SLM andincreases, but less dramatically, for increases in total flow above 10SLM.

Accordingly, when operating at a partial pressure to provide enhancedoxidation, the oxidation rate of silicon can be said to be “masstransport rate” limited. That is, the oxidation rate is limited by theamount of reactant gas fed into the chamber. The fact that the insitusteam oxidation process of the present invention can be “mass transportrate” limited is quite unexpected in that the present invention utilizesrelatively large reactant gas flow rates (greater than 5 SLM into a 2liter chamber) At such high flow rates one would expect there to besufficient reactants available to make the oxidation rate independent ofthe mass transport rate. It is to be appreciated that silicon oxidationprocesses are generally thought to be “surface reaction rate” limitedwhere the temperature controls the oxidation rate and not the flow rateof the reactant gases.

Although the present invention has been described with respect to theinsitu generation of a vapor of a Specific reactive species, water, itis to be appreciated that the teachings of the present invention can-beapplied to other processes where the temperature of a wafer is used toinitiate or catalyze the reaction of reactant gasses to form a vapor ofa reactive species near the wafer surface. The reactive species vaporcan then be reacted with the wafer or with films formed thereon to carryout processes such as film growth. For example, the insitu vaporgeneration process of the present invention can be utilized to convert asilicon dioxide (SiO₂) film into a robust silicon-oxy-nitride film. Forexample, a reactant gas mixture comprising ammonia (NH₃) and oxygen (O₂)can be fed into a chamber and then caused to react by heating a wafer toa sufficient temperature to initiate a reaction of the gasses to formnitric oxide (NO) in vapor form. The nitric oxide vapor can then becaused to react with an oxide film formed on the wafer to form asilicon-oxy-nitride film. Silicon-oxy-nitride films have been found toprovide robust gate dielectric layers at thicknesses less than 100 Å.Other applications for the insitu vapor generation process of thepresent invention will be evident to those skilled in the art.

Thus, a novel method and apparatus for the insitu generation of steam ina rapid thermal oxidation process has been described.

We claim:
 1. A method of forming an oxide comprising: placing asubstrate in a chamber; providing an oxygen containing gas into saidchamber; providing a hydrogen containing gas into said chamber whereinthe combined partial pressure of said hydrogen containing gas and saidoxygen containing gas is less than or equal to 150 torr during saidreaction; reacting said oxygen containing gas and said hydrogencontaining gas in said chamber near said substrate to form an ambient;and oxidizing said substrate with said ambient.
 2. The method of claim 1wherein the combined partial pressure of said hydrogen containing gasand said oxygen containing gas is less than or equal to 50 torr duringsaid reaction.
 3. A method of forming an oxide comprising: placing asubstrate in a chamber; providing an oxygen containing gas into saidchamber; providing a hydrogen containing gas into said chamber whereinthe combined partial pressure of said hydrogen containing gas and saidoxygen containing gas is between 1-50 torr; reacting said oxygencontaining gas and said hydrogen containing gas in said chamber nearsaid substrate to form an ambient; and oxidizing said substrate withsaid ambient.
 4. A method of forming an oxide comprising: placing asubstrate in a chamber; providing an oxygen containing gas into saidchamber wherein said oxygen containing gas is nitrous oxide (N₂O);providing a hydrogen containing gas into said chamber; reacting saidoxygen containing gas and said hydrogen containing gas in said chambernear said substrate to form an ambient; and oxidizing said substratewith said ambient.
 5. A method of forming an oxide comprising: placing asubstrate in a chamber; providing an oxygen containing gas into saidchamber; providing a hydrogen containing gas into said chamber whereinsaid hydrogen containing gas is a hydrocarbon; reacting said oxygencontaining gas and said hydrogen containing gas in said chamber nearsaid substrate to form an ambient; and oxidizing said substrate withsaid ambient.
 6. A method of forming an oxide comprising: placing asubstrate in a chamber; providing an oxygen containing gas into saidcamber; providing a hydrogen containing gas into said chamber whereinsaid hydrogen containing gas is a deuterium; reacting said oxygencontaining gas and said hydrogen containing gas in said chamber nearsaid substrate to form an ambient; and oxidizing said substrate withsaid ambient.
 7. A method of forming an oxide comprising: placing asubstrate having a silicon surface or film in a chamber; providing anoxygen containing gas into said chamber; providing a hydrogen containinggas into said chambers wherein the combined partial pressure of saidhydrogen containing gas and said oxygen containing gas is between 1-50torr; heating said substrate to a temperature sufficient to cause areaction between said hydrogen containing gas and said oxygen containinggas; reacting said oxygen containing gas and said hydrogen containinggas in said chamber to form an ambient wherein said reaction isinitiated by said heated substrate; and oxidizing said silicon surfaceor film with said ambient to form said oxide.
 8. A method of forming anoxide comprising: placing a substrate in a chamber; providing a processgas mix consisting essentially of a hydrogen containing gas and anoxygen containing gas into said chamber; generating a combined partialpressure of said oxygen containing gas and said hydrogen containing gasof less than or equal to 150 torr in said chamber; reacting said oxygencontaining gas and said hydrogen containing gas in said chamber to forman ambient; and oxidizing said substrate with said ambient.
 9. Themethod of claim 8 wherein said combined partial pressure is less than orequal to 50 torr.
 10. The method of claim 8 wherein said combinedpartial pressure is between 1-50 torr.
 11. A method of forming an oxidecomprising: placing a substrate having a silicon film or surface in achamber; providing a process gas mix consisting essentially of hydrogengas (H₂) and oxygen gas (O₂) into said chamber; generating a combinedpartial pressure of said hydrogen gas (H₂) and said oxygen gas (O₂) ofless than or equal to 150 torr in said chamber; reacting said oxygen gasand said hydrogen gas in said chamber to form an ambient in saidchamber; and oxidizing said silicon film or said silicon surface withsaid ambient.
 12. The method of claim 11 wherein said combined partialpressure is less than or equal to 50 torr.
 13. The method of claim 11wherein said combined partial pressure is between 1-50 torr.
 14. Amethod of forming an oxide comprising: placing a substrate into achamber; providing a reactant gas mix comprising a oxygen containing gasand a hydrogen containing gas into said chamber wherein said reactantgas mix has a partial pressure of less than or equal to 30 torr;reacting said oxygen containing gas and said hydrogen containing gas toform an ambient; and oxidizing said substrate with said ambient.
 15. Themethod of claim 14 wherein said partial pressure is less than 15 torr.16. The method of claim 14 wherein said oxygen containing gas is oxygen(O₂) gas and said hydrogen containing gas is hydrogen (H₂) gas andwherein said reactant gas mix is provided into said chamber at a ratiobetween 2% H₂/98% O₂ to 33% H₂/66% O₂.